Modified lithium vanadium oxide electrode materials and products

Abstract

Improved lithium vanadium oxide formulations are presented having a nominal formula of LixV3−&dgr;M&dgr;Oy. Herein preferred cation doped vanadium oxide materials, electrodes using such materials, and electrochemical cells including at least one electrode therein comprising such materials are provided.

Description

FIELD OF THE INVENTION

The present invention relates to certain modified lithium vanadium oxides. Included are preferred oxides according to the general formula LixV3−&dgr;M&dgr;O8, wherein M can be a variety of cations (or a mixture of cations). The invention concerns the utilization of such oxide materials as electrode materials, for example, as cathode materials in lithium batteries. The disclosure concerns preferred formulations of such materials, preferred methods for preparation, products including such materials and methods of use.

BACKGROUND OF THE INVENTION

The negative electrode (anode) of a high density lithium battery typically comprises one or more of a variety of any suitable lithium-containing substances such as: metallic lithium; lithium-metal alloys; lithium metal oxides; or, lithium carbon composites. The positive electrode (cathode) is typically a lithium vanadium oxide of the formula LiV3O8. The electrodes may be coupled using a liquid electrolyte or a solid electrolyte such as a solid polymer electrolyte, or a combination of liquid and solid electrolytes. The electrolyte may specifically be a “plasticized” electrolyte in which a liquid electrolyte component is contained within a polymer electrolyte. During discharge, lithium ions are electrochemically inserted into the lithium vanadium oxide structure by a process that is commonly referred to as intercalation. A reverse process occurs during charge. The vanadium ions of the host electrode structure are reduced and oxidized during discharge and charge, respectively. Conversely, the negative electrode is oxidized during discharge when lithium ions are released from the electrode into the electrolyte, and it is reduced during the reverse process on charge. Lithium ions, therefore, shuttle between the two electrodes during the electrochemical discharge and charge processes.

It is advantageous for batteries, such as lithium batteries, to have a high electrochemical “capacity” or energy storage capability. In lithium batteries, this can be achieved if the positive and negative electrodes can accommodate a significant amount of lithium. Furthermore, in order to have a good cycle life, the positive and negative electrodes should preferably have the ability to accommodate and release lithium in a reversible manner, i.e., without significant “capacity fade.” Thus, preferably, the structural integrity of the electrodes should be maintained during lithium insertion/extraction for numerous cycles.

SUMMARY OF THE INVENTION

According to the present invention, a vanadium oxide material doped with one or more cations is provided. The invention also concerns the provision of electrodes including lithium vanadium oxide according to the preferred general formula; and, batteries including an electrode as characterized.

In one embodiment, the present invention provides a vanadium oxide material according to the average formula:

LixV3−&dgr;M&dgr;Oy

wherein:

(a) 0<&dgr;≦1.0;

(b) 7.8<y≦8.2;

(c) x is non-zero;

(d) x and y are selected such that the average, calculated oxidation state of V is at least 4.7; and

(e) M represents a mixture of at least two different cations.

In a second embodiment, the present invention provides a vanadium oxide material according to the average formula:

LixV3−&dgr;M&dgr;Oy

wherein:

(a) 0<&dgr;≦1.0;

(b) 7.8<y≦8.2;

(c) x is non-zero;

(d) x and y are selected such that the average, calculated oxidation state of V is at least 4.7; and

(e) M represents Mo, Cr, Nb, or mixtures thereof.

In a third embodiment, the present invention provides an electrode having a vanadium oxide material according to the average formula:

LixV3−&dgr;M&dgr;Oy

wherein:

(a) 0<&dgr;≦1.0;

(b) 7.8<y≦8.2;

(c) x is non-zero;

(d) x and y are selected such that the average, calculated oxidation state of V is at least 4.7; and

(e) M represents a mixture of at least two different cations.

In a fourth embodiment, the present invention provides an electrode having a vanadium oxide material according to the average formula:

LixV3−&dgr;M&dgr;Oy

wherein:

(a) 0<&dgr;≦1.0;

(b) 7.8<y≦8.2;

(c) x is non-zero;

(d) x and y are selected such that the average, calculated oxidation state of V is at least 4.7; and

(e) M represents Mo, Cr, Nb, or mixtures thereof.

In a fifth embodiment, the present invention provides an electrochemical cell having a cathode containing a vanadium oxide material according to the average formula:

LixV3−&dgr;M&dgr;Oy

wherein:

(a) 0<&dgr;≦1.0;

(b) 7.8<y≦8.2;

(c) x is non-zero;

(d) x and y are selected such that the average, calculated oxidation state of V is at least 4.7; and

(e) M represents a mixture of at least two different cations.

In a sixth embodiment, the present invention provides an electrochemical cell having a cathode containing a vanadium oxide material according to the average formula:

LixV3−&dgr;M&dgr;Oy

wherein:

(a) 0<&dgr;≦1.0;

(b) 7.8<y≦8.2;

(c) x is non-zero;

(d) x and y are selected such that the average, calculated oxidation state of V is at least 4.7; and

(e) M represents Mo, Cr, Nb, or mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the powder X-ray diffraction pattern of a standard Li1.2V3O8 material.

FIG. 2 is the powder X-ray diffraction pattern of a Li1.2V2.8Ti0.1Mo0.1O8 material.

FIG. 3 is the powder X-ray diffraction pattern of a Li1.2V2.9Ti0.05Mo0.05O8 material.

FIG. 4 is the powder X-ray diffraction pattern of a Li1.2V2.8Zr0.1Mo0.1O8 material.

FIG. 5 is the powder X-ray diffraction pattern of a Li1.2V2.7Y0.1Mo0.2O8 material.

FIG. 6 is the powder X-ray diffraction pattern of a Li1.2V2.7Sc0.1Mo0.2O8 material.

FIG. 7 is the powder X-ray diffraction pattern of a V1.8Ti0.1Mo0.1O5 precursor material.

FIG. 8 is the powder X-ray diffraction pattern of a Li1.2V2.7Ti0.15Mo0.15O8 material derived from the precursor in FIG. 7.

FIG. 9 is a typical voltage profile for a standard Li/Li1.2V3O8.

FIG. 10 is a typical voltage profile for a Li/LixV3−&dgr;M&dgr;Oy cell of the present invention.

FIG. 11 is the capacity vs. cycle number plot for a standard Li/Li1.2V3O8 cell for 20 cycles.

FIG. 12 is the capacity vs. cycle number plot for a Li/Li1.2V2.8Ti0.1Mo0.1O8 cell for 20 cycles.

FIG. 13 is the capacity vs. cycle number plot for a Li/Li1.2V2.9Ti0.05Mo0.05O8 cell for 20 cycles.

FIG. 14 is the capacity vs. cycle number plot for a Li/Li1.2V2.8Zr0.1Mo0.1O8 cell for 20 cycles.

FIG. 15 is the capacity vs. cycle number plot for a Li/Li1.2V2.7Y0.1Mo0.2O8 cell for 20 cycles.

FIG. 16 is a representation of the structure of Li1.2V3O8.

FIG. 17 is a structural representation of a discharged electrode product Li4V3O8.

FIG. 18 is a schematic representation of an electrochemical cell.

FIG. 19 is a second schematic representation of an electrochemical cell.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

I. A General Description of Lithium Vanadium Oxide Electrode Materials

A preferred vanadium oxide electrode material, for use with respect to lithium batteries of concern to the present invention will be referenced generally as having a nominal formula of LixV3Oy, wherein x is non-zero, preferably about 1.0 to about 1.5, and more preferably about 1.2, y is preferably greater than about 7.8 and no greater than about 8.2, and more preferably about 8.0. The crystalline structure of this material is relatively stable, and is favorable with respect to intercalation. This nominal or base formula is the approximate formula at complete charge. Oxides of this nominal formula LixV3Oy exhibit distinctive X-ray diffraction patterns (XRD) and crystalline structures, as discussed below.

The specific preferred stoichiometry for the most stable electrode in the completely charged state is Li1.2V3O8. By this it is meant that the preferred material is formulated from precursor materials such that in a fully charged cell the average formulation of the cathode, with respect to the vanadium oxide component, is Li1.2V3O8. The average (calculated) vanadium valence in Li1.2V3O8 is 4.933 or “nominally” 5.

As the battery is discharged, lithium cations are inserted into the crystalline Li1.2V3O8 electrode structure. This reduces the average oxidation state of the vanadium ions from 4.933 in Li1.2V3O8 to 4.0 in Li4V3O8, which represents the approximate composition of the positive electrode in a discharged cell.

According to the present invention, the nominal LixV3Oy structure (wherein x and y are as defined above) typically and preferably Li1.2V3O8, is modified to advantage. The modification, in part, concerns “doping” the crystalline structure with one or more cations.

II. Preferred Modified Electrode Materials

In further embodiments of the present invention, substitution of vanadium by another element, preferably a cation in addition to lithium, is used. This can also be a method of maintaining the average, calculated oxidation state of vanadium at a value of at least 4.7 (preferably at least 4.8, more preferably at least 4.85, even more preferably at least 4.9, and most preferably at or near 4.933) in the fully charged state. By definition the oxidation state of vanadium is no greater than 5.0, and preferably no greater than 4.95.

Substitution of one or more (preferably, two or more) cations, particularly one or more (preferably, two or more) metal cations, for some of the vanadium in the material results in a general formula as follows: LixV3−&dgr;M&dgr;Oy, wherein x and y are as defined above, and &dgr; is greater than zero and typically no greater than about 1.0. A base formula for a preferred group of such stabilized compounds would be as follows: Li1.2V3−&dgr;M&dgr;O8 wherein M is a cation (or mixture of two or more cations). Suitable cations are those that are sufficiently small such that they can fit into sites previously occupied by vanadium (typically, octahedral sites) and/or sites previously occupied by lithium (typically, octahedral or tetrahedral sites). In general, it is preferred to use a cation M (or mixture of two or more cations M), which does not, in the amounts used, generate a significant amount of a second impurity phase along with Li1.2V3O8.

Preferred cations, M, are those selected from Mg, Al, Si, P, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Ta, Mo, La, Hf, W, or mixtures thereof. More preferred cations, M, are those that form strong M—O bonds, such as Mg, Al, Si, Sc, Ti, Y, Zr, Mo, or mixtures thereof. For certain particularly preferred embodiments, the cations, M, are Mo, Cr, Nb, or mixtures thereof. For certain other particularly preferred embodiments, the cations, M, are Mg, Al, Y, Ti, Zr, Mo, or mixtures thereof. Of these, titanium and/or zirconium are particularly preferred, although other cations and even mixtures, can be used. The titanium or zirconium doped systems are advantageous because of the relatively strong titanium-oxygen or zirconium-oxygen bonds in the crystal structure. It can be reasoned that such bonds will serve to strengthen and maintain the integrity of the overall crystal structure, particularly during cell operation when lithium is being repeatedly inserted into and extracted from the structure, and to suppress oxygen loss from the structure as the electrode approaches the fully charged state.

In various embodiments of the present invention, it is possible to use two or more different cations as substituents for some of the vanadium ions. For example, it is possible to replace two pentavalent vanadium ions (V5+) in the lithium-vanadium-oxide crystal lattice by one hexavalent molybdenum (Mo6+) ion and one tetravalent zirconium (Zr4+) or titanium (Ti4+) ion. In this instance, substitution of two metal cations for some of the vanadium in the material results in the general formula: Li1.2V3−2&dgr;M′&dgr;M″&dgr;O8. Thus, as an example, for M′=Mo+6, M″=Ti+4, &dgr;=0.1, a preferred formula would be:

Li1.2V2.8Mo0.1Ti0.1O8.0.

III. Methods of Preparation

Materials according to the present invention can be readily prepared by modifications of known techniques for the manufacture of Li1.2V3O8. For example, Li1.2V3O8 electrode materials can be prepared by mixing LiOH.H2O with NH4VO3 and suspending the mixture in methanol to form a reaction mixture. This reaction mixture is preferably subsequently milled for a time period of about 24 hours to about 48 hours, and the remaining methanol removed (e.g., by evaporation), resulting in a dry solid precursor of Li1.2V3O8. The precursor can then be heated to a temperature of about 20° C. to about 400° C., at a heating rate of about 1° C./minute for a time period of about 24 hours, and then cooled to room temperature at a cooling rate of about 1° C./minute. The resulting product may then be ground to a fine powder, for example, by high energy ball milling during which the powders are agitated at high frequency (for example, in a spherical stainless steel container with one steel grinding ball in a Spex #8000D Miller/Mixer (Metuchen, N.J.)) for about 96 hours or less. This milling process reduces the particle size significantly and eliminates the need for fluorine doping to reduce particle size.

In addition to employing vanadium cations in preparing materials of the invention, other metal cations may also be introduced into the crystal structure of the lithium vanadium oxide material. For example, titanium may be introduced into the reaction mixture prepared above by the further addition of Ti[OCH(CH3)2]4. Similarly, zirconium and molybdenum may also be introduced to the reaction mixture by the addition of Zr[OCH(CH3)2]4. (CH3)2CHOH and MoO3, respectively. Additionally, other metal cations that may be added to the reaction mixture include yttrium and scandium. Yttrium may be introduced into the reaction mixture by the addition of Y5O[OCH(CH3)2]13. Scandium may be introduced into the reaction mixture by the addition of Sc[OCH(CH3)2]3. For other cations, the precursor compounds can be selected from oxides, hydroxides, alkoxides, oxalates, acetates, nitrates, or mixtures thereof.

Electrodes can be prepared from the oxide base material by coating onto a metallic current collector a slurry containing the oxide base, a polymeric binder such as polyvinylidinefluoride (PVDF), an electrically conductive. particle such as conductive carbon particles, and a solvent such as toluene. This coating is then dried to form the electrode.

Advantageously, preparation of the materials according to the present invention may be accomplished without fluorine doping and with surprising stability.

IV. Some Theories for Certain Observed Stabilization Effects

Electrode materials of the type described herein would typically be used in high energy density lithium storage batteries. The capacity fade that occurs with cycling for certain types of such batteries (repeated discharge and charge) can be attributed to a number of possible factors. Li1.2V3O8 has a layered-type crystal structure. During discharge, lithium ions are inserted into a Li1.2+x′V3O8 structure (typically 0≦x′≦2.8). Capacity fade phenomena could result, for example, either from (1) structural fatigue due to anisotropic variations in the lattice parameters during charge and discharge, or (2) migration of vanadium ions from their original positions into the layers occupied by lithium, thereby, restricting lithium mobility, or (3) the dissolution of some vanadium containing species from the crystal lattice into the electrolyte, or (4) a loss of oxygen from the electrode structure at or near the fully charged state, or various combinations of (1), (2), (3), and (4).

In preferred lithium vanadium oxide cathode arrangements according to the present invention, at the “top of the charge,” the vanadium is in an average oxidation state approaching V5+ (typically and preferably about 4.933, more generally at least 4.7) and at the end of discharge it is closer to V4+. Vanadium in lower oxidation states (such as V3+) is believed to be somewhat more soluble in certain electrolytes than at higher oxidation states (such as V4+). This could be partly responsible for some of the observed deterioration of the cathode operation, with cycling for certain types of batteries.

Without being bound to any particular theory, it is presently believed that the M-cation doped systems are stabilized by introduction of the M cations into the crystal structure. Related samples with fluorine doping (i.e., substitution of fluoride ions for oxygen ions in the crystal structure) disclosed in U.S. patent application Ser. No. 08/985,441 (filed Dec. 5, 1997) have previously shown improved electrochemical performance, the reasons for which are now more completely understood. Scanning electron microscopy revealed that one feature of fluorine doping was to reduce the particle size of the standard Li1.2V3O8 samples. It has now been discovered that this physical property of the material can be achieved by alternative processing methods without the necessary use of fluorine, for example by high energy ball milling of the Li1.2V3O8 to reduce particle size. Furthermore, using fluorine-based materials in laboratory and production processes is generally neither desirable nor advantageous because of their high chemical reactivity and the special handling procedures typically required. It is now believed that the reason for the improved stability of the metal oxide framework by transition metal substitution for vanadium can be attributed to differences in strength between vanadium-oxygen and M-oxygen bonds, although the invention is not necessarily limited by this theory. Therefore, it is believed that the introduction of M cations into the structure, in the absence of fluorine, may add integrity to the vanadium oxide crystalline structure, as a result of the introduction of strong M-oxygen bonds. The net result of this could be inhibiting vanadium migration, inhibiting solubility, and/or suppressing oxygen loss at the top of charge, although this should not be limiting to the invention. Thus, in some instances, the slight modification to the electrode composition and crystal structure may manifest itself by a lessening of capacity fade.

In general, the preferred formulations provided are arrived at by focusing on two principal factors:

1. A desire to maintain the vanadium valence state at the top of charge, as close to 4.933 as reasonably possible, and more generally at least 4.7 (preferably at least 4.8, more preferably at least 4.85, even more preferably at least 4.9, and most preferably at or near 4.933) in the stabilized LixV3Oy (preferably Li1.2V3O8) crystal structure; and

2. Introduction of no more M cations into the crystalline structure than is useful to achieve the desired level of stabilization, because of a desire not to greatly depart from the stoichiometry of the Li1.2V3O8 base (i.e., to maximize the available capacity of the electrode), and to avoid the use of fluorine (a chemically aggressive reagent) in the oxide.

In general, the crystalline structure of Li1.2V3O8 is layered. In the standard structure, three vanadium ions and one lithium ion typically occupy octahedral sites in the Li1.2V3O8 structure; the remaining 0.2 lithium ions occupy tetrahedral sites. During discharge, the lithium ions migrate into neighboring octahedral sites to generate a stable defect rocksalt structure, Li4V3O8, which is the approximate composition at the end of discharge. The stoichiometric rocksalt composition, Li5V3O8, is not easily attainable at the end of the electrochemical discharge.

V. General Formulae of Preferred Materials Based on the Above-Recited Principles and Descriptions.

A general formula of a preferred vanadium oxide material, useable as a cathode material as described above at least when defined in the charged state, would be as follows:

LixV3−&dgr;M&dgr;Oy

wherein preferably:

(a) 0<&dgr;≦1.0 (more preferably 0.05≦&dgr;≦0.3);

(b) 7.8<y≦8.2;

(c) x is non-zero (typically and preferably 1.0≦x≦1.5 and more preferably x is about 1.2);

(d) x and y are selected such that the average, calculated oxidation state of V is at least 4.7; and,

(e) M represents a cation (preferably, at least two different cations). Suitable cations are those that are sufficiently small that they can fit into sites previously occupied by vanadium (typically, octahedral sites) and/or sites previously occupied by lithium (typically, octahedral or tetrahedral sites).

Preferred electrodes comprise a vanadium oxide material according to the formulae recited above; and, preferred battery constructions include at least one preferred electrode as characterized. The values of x, &dgr;, and y are average values.

It should be appreciated that in some instances M may be a mixture of cations and thus the term “M&dgr;” is intended to include mixtures of cations. In such instances the limitation on “&dgr;” is intended to be on the averaged cation “M” resulting from averaging the valence of the various M′, M″, etc., using a mole-weighted, valence-charge-balance formula consistent with the general formula LixV3−&dgr;M&dgr;Oy, Particularly preferred electrodes which contain a mixture of cations are those in which M is derived from Mg, Al, Y, Ti, Zr, and Mo.

As indicated above, the formulae given herein for the preferred vanadium oxide materials are generally in reference to the material as it would be found in an electrode in the fully charged state (i.e., upon initial synthesis of the material). During discharge, and intercalation, a lithium ion introduction will modify the formulae.

VI. An Example of Battery Construction

Typically battery constructions that can use the preferred cathode materials according to the present invention include otherwise conventional coin cells, wound cells, and stacked cells in which the cathode oxide is replaced with the preferred oxide(s) described herein. Various useable conventional constructions are described in Handbook of Batteries, 2d Ed., edited by D. Linden et al., McGraw-Hill, 1995.

An example cell is shown in FIG. 18. The cell may generally be made according to the description of U.S. Pat. No. 4,803,137 (Mayazaki et al.), except in that the cathode includes a vanadium oxide material as described herein. Referring to FIG. 18, the cell depicted includes: a cathode 1; a positive electrode current collector 2; a positive electrode casing 3; an anode 4; a negative electrode current collector 5; a negative electrode casing 6; separator/electrolyte 7; and, insulating polypropylene gasket 8. With a vanadium oxide material as described herein, the cell would operate in the otherwise typical fashion.

Another schematic illustration of the electrochemical cell is shown in FIG. 19. The cell is designated 15, and the anode (negative electrode), electrolyte and cathode (positive electrode) are designated 11, 12, and 13, respectively, with the anode 11 separated from the cathode 13 by the electrolyte 12. Suitable terminals designated 14 are provided in electronic contact with the anode 11 and the cathode 13. The cell 15 is contained in a housing, designated 16, which insulates the anode from the cathode. The cell 15 may include, at the cathode 13, vanadium oxide material according to the present invention.

Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.

Experimental

A. Preparation of Materials

Li1.2V3O8

1.2 moles of LiOH.H2O (Aldrich Chemical Co., Milwaukee, Wis.) and 3 moles of NH4VO3 (Aldrich) were suspended in methanol and milled in a Sweco (paint shaker) mill (Sweco, Florence, Ky.) in a polyethylene container with yttria stabilized ZrO2 grinding spheres (Tosoh Corporation, Tokyo, Japan) for 48 hours. The methanol was evaporated by heating above 70° C. in a fumehood and the dry solid precursor was heat treated in air. The sample was heated to 400° C. at a rate of 1° C./minute, and held at 400° C. for 24 hours, then allowed to cool to room temperature (1° C./minute cooling rate). The product was ground manually to a fine powder and submitted for phase identification by powder X-ray diffraction on a Seimens D-5000 diffractometer (Madison, Wis.) (FIG. 1).

Li1.2V2.8Ti0.lMo0.1O8

1.2 moles of LiOH.H2O, 2.8 moles of NH4VO3, 0.1 mole of Ti[OCH(CH3)2]4 (Aldrich), and 0.1 mole of MoO3 (Aldrich) were suspended in methanol and milled as described above for 48 hours. The methanol was evaporated by heating above 70° C. in a fumehood and the dry white solid precursor was heat treated in air. The sample was heated to 400° C. at a rate of 1° C./minute, and held at 400° C. for 24 hours, then allowed to cool to room temperature (1° C./minute cooling rate). The product was ground manually to a fine powder and submitted for phase identification by powder X-ray diffraction on a Seimens D-5000 diffractometer (FIG. 2).

Li1.2V2.9Ti0.05Mo0.05O8

1.2 moles of LiOH.H2O, 2.9 moles of NH4VO3, 0.05 mole of Ti[OCH(CH3)2]4, and 0.05 mole of MoO3 were suspended in methanol and milled as described above for 48 hours. The methanol was evaporated by heating above 70° C. in a fumehood and the dry white solid precursor was heat treated in air. The sample was heated to 400° C. at a rate of 1° C./minute, and held at 400° C. for 24 hours, then allowed to cool to room temperature (1° C./minute cooling rate). The product was ground manually to a fine powder and submitted for phase identification by powder X-ray diffraction on a Seimens D-5000 diffractometer (FIG. 3).

Li1.2V2.8Zr0.1Mo0.1O8

1.2 moles of LiOH.H2O, 2.8 moles of NH4VO3, 0.1 mole of Zr[OCH(CH3)2]4(CH3)2CHOH (Aldrich), and 0.1 mole of MoO3 were suspended in methanol and milled as described above for 48 hours. The methanol was evaporated in a fumehood above 70° C. and the dry white solid precursor was heat treated in air. The sample was heated to 400° C. at a rate of 1° C./minute, and held at 400° C. for 24 hours, then allowed to cool to room temperature (1° C./minute cooling rate). The product was ground manually to a fine powder and submitted for phase identification by powder X-ray diffraction on a Seimens D-5000 diffractometer (FIG. 4).

Li1.2V2.7Y0.1Mo0.2O8

1.2 moles of LiOH.H2O, 2.7 moles of NH4VO3, 0.02 mole of Y5O[OCH(CH3)2]13 (Chemat, Northbridge, Calif.), and 0.2 moles of MoO3 were suspended in methanol and milled as described above for 48 hours. The methanol was evaporated in a fumehood above 70° C. and the dry white solid precursor was heat treated in air. The sample was heated to 400° C. at a rate of 1° C./minute, and held at 400° C. for 24 hours, then allowed to cool to room temperature (1° C./minute cooling rate). The product was ground manually to a fine powder and submitted for phase identification by powder X-ray diffraction on a Seimens D-5000 diffractometer (FIG. 5).

Li1.2V2.7Sc0.1Mo0.2O8

1.2 moles of LiOH.H2O, 2.7 moles of NH4VO3, 0.1 mole of Sc[OCH(CH3)2]3 (Chemat, Northbridge, Calif.), and 0.2 mole of MoO3 were suspended in methanol and milled as described above for 48 hours. The methanol was evaporated in a above 70° C. and the dry white solid precursor was heat treated in air. The sample was heated to 400° C. at a rate of 1° C./minute, and held at 400° C. for 24 hours, then allowed to cool to room temperature (1° C./minute cooling rate). The product was ground manually to a fine powder and submitted for phase identification by powder X-ray diffraction on a Seimens D-5000 diffractometer (FIG. 6).

B. Alternative Synthesis for Molybdenum and Group IV Transition Metal Doping of Lil.2V3O8 Structure

An alternative method to the synthesis described above in part A, is a synthesis having a two step process which involves the preparation of a vanadium-molybdenum-group IV oxide precursor, such as a vanadium-molybdenum-titanium precursor, which is then reacted with a lithium containing reagent.

V2-2xTixMoxO5 Precursor

1.8 moles of NH4VO3, 0.1 mole of Ti[OCH(CH3)2]4, and 0.1 mole of MoO3 were suspended in methanol and milled as described above for 48 hours. The methanol was evaporated in a fumehood above 70° C. and the solid was heated in air. The sample was subsequently heated to 600° C. at a rate of 1° C./minute, and held at 600° C. for 24 hours, then allowed to cool to room temperature (1° C./minute cooling rate). The product was ground manually to a fine powder and submitted for phase identification by powder X-ray diffraction on a Seimens D-5000 diffractometer (FIG. 7).

Li1.2V2.7Ti0.15Mo0.15O8

1.2 moles of LiOH.H2O and 1.5 moles of V2-2xTixMoxO5 (as prepared above with x=0.1) were suspended in methanol and milled as described above for 48 hours. The methanol was evaporated in a fumehood above 70° C. and the solid was heat treated in air. The sample was heated to 400° C. at a rate of 1° C./minute, and held at 400° C. for 24 hours, then allowed to cool to room temperature (1° C./minute cooling rate). The product was ground manually to a fine powder and submitted for phase identification by powder X-ray diffraction on a Seimens D-5000 diffractometer (FIG. 8).

C. Electrochemical Testing

The materials prepared above were screened for electrochemical performance in “1225” or “2032” coin cells (12 mm×2.5 mm high and 20 mm×3.2 mm high, respectively). Blended materials for cathode laminates were prepared by: 1) mixing by weight 81% active material, 10% KYNAR (binder) (Elf-Atochem, Philadelphia, Pa.), and 9% carbon (Cabot Corporation, Boston, Mass.); and 2) ball milling the materials in a Sweco (paint shaker) mill (Sweco, Florence, Ky.) in tetrahydrofuran (Aldrich) or 1-methyl-2-pyrrolidinone (Aldrich) with yttria stabilized ZrO2 grinding spheres (Tosoh Corporation, Tokyo, Japan).

Laminates were prepared by a doctor blade, whereby a slurry of the blended materials described above were evenly coated onto a thin Al foil about 21 microns thick, and thereafter dried overnight in a vacuum oven at 80° C. The electrolyte used for electrochemical evaluations was a 1 Molar solution of LiPF6 (Kerr-McGee, Oklahoma City, Okla.) dissolved in a 50:50 mixture (by volume) of dimethyl carbonate (DMC) and ethylene carbonate (EC) (Kerr-McGee, Oklahoma City, Okla.). Li/1.0M LiPF6, DMC, EC/LixV3−&dgr;M&dgr;Oy cells were cycled at constant current (typically 0.1 milliamp (mA)) between 3.1-2.1 volts (V).

Electrochemical Data

A typical voltage profile that is obtained during cycling of a standard Li/Li1.2V3O8 cell is provided in FIG. 9. A typical voltage profile of a typical Li/LixV3−&dgr;M&dgr;Oy cell of the present invention is provided in FIG. 10.

Individual plots of discharge capacity vs. cycle number for a standard Li/Li1.2V3O8 cell, for a Li/Li1.2V2.8Ti0.1Mo0.1O8 cell, for a Li/Li1.2V2.9Ti0.05Mo0.05O8 cell, for a Li/Li1.2V2.8Zr0.1Mo0.1O8 cell, and for a Li/Li1.2V2.7Y0.1Mo0.02O8 cell in accordance with this invention are shown in FIGS. 11, 12, 13, 14 and 15, respectively. It was usually found that one “break-in” cycle with a relatively high initial discharge capacity was necessary before the electrode provided stable electrochemical cycling. The superior cycling stability of the LixV3−&dgr;M&dgr;Oy electrodes of the present invention compared to a standard Li1.2V3O8 electrode after the first cycle is clearly evident in FIGS. 11 to 15. This is also demonstrated in Table 1, in which the electrode capacities delivered at cycle 2 and at cycle 20 are listed as well as the overall capacity fade (as a percentage) between cycles 2 and 20 for various Li/LixV3−&dgr;M&dgr;Oy cells.

TABLE 1 Performance of LixV3-&dgr;M&dgr;Oy electrodes in Li/LixV3-&dgr;M&dgr;Oy cells Capacity Capacity Capacity Fade (mAh/g) (mAh/g) (%) Electrode Material Cycle 2 Cycle 20 (Cycle 2 to 20) Li1.2V3O8(control) 208 162 24 Li1.2V2.8Ti0.1Mo0.1O8 226 209 5 Li1.2V2.9Ti0.05Mo0.05O8 229 204 11 Li1.2V2.8Zr0.1Mo0.1O8 181 165 9 Li1.2V2.7Y0.1Mo0.2O8 180 156 13 Li1.2V2.7Sc0.1Mo02O8 177 158 11

The complete disclosures of the patents, patent documents, and publications cited herein are incorporated by reference in their entirety as if each were individually incorporated. Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows.

Claims

1. A vanadium oxide material according to the average formula:

(a) 0<&dgr;&lE;1.0;

(b) 7.8<y&lE;8.2;

(c) x is non-zero;

(d) x and y are selected such that the average, calculated oxidation state of V is at least 4.7; and

(e) M represents a mixture of at least two different cations.

2. A vanadium oxide material according to claim 1, wherein M is selected from the group consisting of Mg, Al, Si, P, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Ta, Mo, La, Hf, W, and mixtures thereof.

3. A vanadium oxide material according to claim 2, wherein M is selected from the group consisting of Mg, Al, Si, Sc, Ti, Y, Zr, Mo, and mixtures thereof.

4. A vanadium oxide material according to claim 3, wherein M is selected from the group consisting of Mg, Al, Ti, Y, Zr, Mo, and mixtures thereof.

5. A vanadium oxide material according to claim 1, where in the average, calculated oxidation state of V is at least 4.85.

6. A vanadium oxide material according to claim 1, wherein x is about 1.0 to about 1.5.

7. An electrode comprising a vanadium oxide material according to the average formula:

(a) 0<&dgr;&lE;1.0;

(b) 7.8<y&lE;8.2;

(c) x is non-zero;

(d) x and y are selected such that the average, calculated oxidation state of V is at least 4.7; and

(e) M represents a mixture of at least two different cations.

8. An electrode according to claim 7, wherein the average, calculated oxidation state of V is at least 4.85.

9. An electrode according to claim 7, wherein M is selected from the group consisting of Mg, Al, Si, P, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Ta, Mo, La, Hf, W, and mixtures thereof.

10. An electrode according to claim 9, wherein M is selected from the group consisting of Mg, Al, Si, Sc, Ti, Y, Zr, Mo, and mixtures thereof.

11. It An electrode according to claim 10, wherein M is selected from the group consisting of Mg, Al, Ti, Y, Zr, Mo, and mixtures thereof.

12. An electrode according to claim 7, further comprising a binder and conductive material.

13. An electrochemical cell comprising a cathode comprising a vanadium oxide material according to the average formula:

(a) 0<&dgr;&lE;1.0;

(b) 7.8<y&lE;8.2;

(c) x is non-zero;

(d) x and y are selected such that the average, calculated oxidation state of V is at least 4.7; and

(e) M represents a mixture of at least two different cations.

14. An electrochemical cell according to claim 13, wherein M is selected from the group Mg, Al, Si, P, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Ta, Mo, La, Hf, W, and mixtures thereof.

15. An electrochemical cell according to claim 14 wherein M is selected from the group consisting of Mg, Al, Si, Sc, Ti, Y, Zr, Mo, and mixtures thereof.

16. An electrochemical cell according to claim 15 wherein M is selected from the group consisting of Mg, Al, Ti, Y, Zr, Mo, and mixtures thereof.

17. An electrochemical cell according to claim 16 wherein M comprises Ti, Zr, or a mixture thereof.